1. Field of the Invention
The invention relates in general to a method of fabricating high density integrated circuits, and more particularly to a method of fabricating dynamic random access memories (DRAM) with a thin film transistor (TFT) to increase the reading and writing speed.
2. Description of the Related Art
The higher the integration of memory devices is, the better the storage ability of a wafer is. In addition, the fabricating cost of a high integration memory device is cheaper than the fabricating cost of a low integration memory device. Therefore, the VLSI process usually provides as high a degree of integration as possible to increase the data storage ability of the wafer.
A conventional method of increasing the density of the memory devices of integrated circuits is to decrease part of the structure size of the memory devices of integrated circuits. This may be accomplished by a decrease in the size of the wiring lines, the size of the gate of the transistor or the size of the isolation region between memory devices. To decrease the size of the memory devices of integrated circuits means to decrease the design rules of the integrated circuits.
Generally speaking, a DRAM cell includes a transfer transistor (or a field effect transistor) and a capacitor. Conventional method of storing the DRAM data is by selectively charging or discharging each of the capacitors formed on the semiconductor substrate to store the DRAM data. That is, a two-bit data can be stored in a capacitor by using the discharging state indicated by logic xe2x80x9c0xe2x80x9d or the charging state indicated by logic xe2x80x9c1xe2x80x9d. The steps of the method of storing the DRAM data includes providing electrical voltage to the gate of the transistor to store charges in the capacitor. The amount of charge stored in the capacitor depends upon the surface of the capacitor electrode, the dielectric coefficient of the dielectric layer of the capacitor and the distance between the upper electrode and the lower electrode (e.g. the thickness of the dielectric layer). The reading operation of the DRAM is performed by using the field effect transistor (FET) to selectively couple the storage capacitor to the bit line, and to transfer charges to the capacitor or retrieve charges from the capacitor. The contact formed between the bit line and the FET is one of the source/drain electrodes of the FET, and another source/drain electrode of the FET is connected to the capacitor. Accordingly, the bit line is connected to the lower electrode of the capacitor via the FET. The signal of the word line is provided from the gate of the FET to transfer the charges between the bit line and the storage capacitor.
As the size of the DRAM minimizes according to the design rule, the surface of the lower electrode of the capacitor decreases. The conventional capacitor is planar in structure. The shrinkage of the surface of the lower electrode of the capacitor thus decreases the amount of the storage charges (e.g. the capacitance). The decreasing of the amount of the storage charges will result in some problems, such as decay mechanisms and charge leakage that result in data loss. Therefore, the capacitor must frequently receive supplementary charge to maintain the amount of the storage charges in the capacitor to resolve the problem of the charge leakage or data loss. The supplementary charges to the capacitor are called refresh cycles. During refresh cycles, the DRAM cannot store or read data. If the charge storage performance decreases, it is necessary to provide a more complicated refresh cycle operation or a more sensitive charge sensor amplifier. Therefore, as the size of the DRAM minimizes according to the design rule, it is necessary that the surface of the lower electrode of the capacitor increase to overcome the problem of charge leakage or data loss.
FIGS. 1A-1C are schematic, cross-sectional views showing a conventional process of fabricating a DRAM. As shown in FIG. 1A, a shallow trench isolation (STI) structure 102 is formed on a substrate 100. A gate oxide layer 104 and a gate electrode 106 are formed on the substrate 100. A source/drain region 108 is formed in the substrate 100 and beside the gate electrode 106. An isolating layer 110, for example, an oxide layer, is formed on the gate electrode 106 and the substrate 100. The isolating layer 110 is patterned to form a contact opening 112.
As shown in FIG. 1B, a conductive layer 114 is formed on the isolating layer 110 and in the contact opening 112 to form a contact 112a that electrically connects one of the source/drain regions 108.
As shown in FIG. 1C, the conductive layer 114 is patterned to form a conductive layer 114a. A hemispherical grain (HSG) layer 115 is formed on the conductive layer 114a. Accordingly, the contact 112a, the conductive layer 114a and the HSG layer 115 together form a lower electrode of a capacitor. A dielectric layer 116 is formed on the HSG layer 115. A conductive layer 118 is formed on the dielectric layer 116 to form an upper electrode of the capacitor.
As the size of the DRAM is minimized according to the design rule, the surface of the lower electrode of the capacitor decreases. The sensitive range of the DRAM and the speed of reading or writing are all poor. It is necessary to provide a method to improve the sensitive range of the DRAM and the speed of reading or writing.
It is an object of the invention to provide a method of fabricating DRAM with a thin film transistor (TFT) to increase the reading and writing speed.
It is another object of the invention to provide a method of fabricating a DRAM with a thin film transistor (TFT). If the voltage of the charge storage is higher than the threshold voltage of the thin film transistor (TFT), the data of charging state xe2x80x9c1xe2x80x9d can be read correctly. Because the thin film transistor (TFT) can provide enough charges to the circuit, the sensitive range of the DRAM improves and the speed of reading and writing improves.
A method of the process according to the invention comprises the following steps. A substrate with a transfer transistor, a word line, a bit line and an interlayer dielectric layer is provided. A bit line contact and a terminal contact are formed in the interlayer dielectric layer. The bit line contact couples to the bit line. The terminal contact couples to the transfer transistor. The terminal contact is a T-shaped structure. An oxide layer is formed to cover the interlayer dielectric layer and the terminal contact and to expose the bit line contact. A polysilicon layer is formed to cover the oxide layer and the bit line contact. An ion implantation step is performed to form a first doped region and a second doped region in the polysilicon layer. The polysilicon layer is patterned to make the first doped region into a source region and to make the second doped region into a drain region. The drain region is on one side of the terminal contact corresponding to the source region.